Long-term fertility experiments for irrigated rice in the West African Sahel: agronomic results

Long-term fertility experiments (LTFEs) are a tool to investigate the sustainability of cropping systems. Several LTFEs for intensive irrigated rice cropping were established in Asia, but those are rare in Sub-Saharan Africa. Two such trials are presented, both located in the Sahel savanna vegetation zone in Senegal. The trials were established in 1991, contain six different fertilizer treatments and rice is grown two times per year. Soil type at Ndiaye is a typical Orthithionic Gleysol and an Eutric Vertisol at Fanaye. Average grain yields without fertilizer application were 3.4 Mg ha −1 per season in Ndiaye and 2.9 Mg ha −1 per season in Fanaye. In 20 consecutive seasons best treatments at both sites and in both seasons yielded on average between 6.7 and 7.6 Mg ha −1 per crop. Yield components were influenced by cultivar, site, season and fertilizer treatment. The mineral composition of grains was homogenous between cultivars and similar to results from Asia, Australia and the USA. Significant differences among cultivars were found for the mineral composition of straw, especially for Si, Fe, Zn and K. Nitrogen, P and K fertilizer treatments increased the mineral concentrations of N, P, Mg, Ca, Mn and Cu. Highest yields in the LTFE indicate a not significant yield decline of −27 kg per season in Ndiaye and a significant increase in Fanaye (+86 kg per season). The crop model ORYZAS simulated that potential yields declined by −50 kg per season in Ndiaye and increased by +16 kg per season in Fanaye (not significant). Simulated yield trends were not always mirrored by trends of average seasonal radiation, which is due to the influence of unfavorable temperature extremes on yield and yield simulations. It is concluded that observed yield trends can largely be explained by climatic influences. The agronomic analysis did not indicate a negative impact of intensive irrigated rice cropping on the soil resource base.